In recent years, micro-fabrication technology on wafers has been requested along with development of higher integration and higher sophistication in VLSI (very large scale integration), and photolithographic technology has been widely used as a fabrication method therein. It is desirable that a projection lens of a projection exposure system, which is a key to the photolithographic technology, possesses high imaging performances (resolution and focal depth).
The resolution and the focal depth depend on a wavelength of light and on an NA (numerical aperture) of a lens which are used upon exposure. When an exposure wavelength λ is constant, an angle of diffraction light grows larger as a pattern becomes finer. Accordingly, it is not possible to pick up the diffraction light unless the NA of the lens is large. Meanwhile, when the exposure wavelength λ is short, the angle of the diffraction light in the same pattern becomes small. Accordingly, the NA of the lens can be made small.
The resolution and the focal depth are severally represented by the following formulae (2) and (3), namely,Resolution=k1·λ/NA  (2)Focal depth=k2·λ/(NA)2  (3)(in the formulae (2) and (3), k1 and k2 severally denote proportionality constants).
From the formula (2), it is learned that either the NA of the lens should be increased (that is, a diameter of the lens is increased) or the exposure wavelength λ should be shortened in order to enhance the resolution, however, it can be said from the formula (3) that shortening λ is particularly advantageous from the viewpoint of the focal depth.
Due to the foregoing reason, recently the exposure wavelength is gradually shortened, and a projection exposure system adopting a light source such as a KrF excimer laser (248-nm wavelength) or an ArF excimer laser (193-nm wavelength) is launched in the market. In such a system, optical materials usable for photolithography with a wavelength at 250 nm or less are extremely limited, and almost all optical systems are designed to use two types of materials which are calcium fluoride and fused silica.
Moreover, practical application of a projection exposure system using an F2 laser (157-nm wavelength) is under consideration in order to exploit further shortening of the exposure wavelength. It is considered that materials usable under this wavelength are limited to certain fluoride crystals beside calcium fluoride, which are strontium fluoride, barium fluoride, lithium fluoride and the like.
Incidentally, it is known that an absorption band is generated in a single crystal of calcium fluoride when a laser beam with high photon energy is irradiated onto the single crystal of calcium fluoride containing a large degree of impurities. If an optical member using such a material is applied to an optical system, there is a case of incurring deterioration of transmittance in the used wavelength under the influence of the absorption band thus generated. Therefore, there is disclosed a use of a single crystal of calcium fluoride having high transmittance and durability with respect to an irradiated laser beam as calcium fluoride for photolithography (Japanese Patent Laid-Open No. 11(1999)-60382 and the like).
Next, speaking of an increase in the diameter of the lens, an optical material for use in photolithography which requires extremely high-level imaging performance is not only satisfied with a large diameter, but it is also essential that the optical material possesses small birefringence and excellent homogeneity in an internal refractive index.
The Bridgman method is generally used as the method of manufacturing calcium fluoride. Upon fabrication of an optical member from a calcium fluoride crystal ingot obtained by the Bridgman method, although there is a case of carving the optical member (material) of a targeted size directly out of the ingot, there is also a case of cutting the ingot into a plurality of blocks and then subjecting the blocks to a heat treating step so as to enhance internal quality such as birefringence and homogeneity in refractive index. For example, Japanese Patent Laid-Open No. 11(1999)-240798 discloses a method of manufacturing a single crystal of calcium fluoride, which has characteristics of birefringence in the direction of the optical axis at 2 nm/cm or below, birefringence in the lateral direction (the direction of an in-plane diameter perpendicular to the optical axis) at 5 nm/cm or below, and a refractive index difference An at 2×10−6 or below.